This invention relates to a solid state image sensor used in a video camera and the like.
A charge coupled device (CCD) solid state image sensor, in particular, an interline transfer type CCD solid state image sensor possesses an excellent resolution and a high photo-sensitivity, and hence it has come to be widely used in video camera and other devices.
FIG. 6 is a sectional view of a unit cell of a conventional interline transfer type CCD image sensor using a PN junction photo diode as a photo detector.
In FIG. 6, one unit cell of a solid state image sensor is composed of a photo detector (PD) and a charge reading unit (CCD). A P-type well 2 is formed on an N-type silicon substrate 1. Near the surface in the P-type well 2, P-type channel stop regions 3, 4 for isolating the adjacent unit cells are formed. These channel stop regions 3, 4 are connected to the ground potential. Closely adjacent to one channel stop region 3 in the P-type well 2, an N-type region 5 is formed by diffusing an N-type impurity, and a PN junction photo diode of the photo detector (PD) is formed of the P-type well 2 and N-type region 5. Closely adjacent to the other channel stop region 4 in the P-type well 2, an N-type region 6 is formed by diffusing an N-type impurity. This N-type region 6 is used as the CCD transfer channel for sequentially transferring the signal electric charges generated in the photo detector (PD). The entire surface of the P-well 2 is covered with a silicon dioxide film 7. A polycrystalline silicon electrode 8 is buried in this silicon dioxide film 7. The polycrystalline silicone electrode 8 serves as both a reading gate for reading signal electric charges from the N-type region 5, to the N-type region 6, and a CCD transfer gate. On the surface of the silicon dioxide film 7, an aluminum film 9 is formed. The aluminum film 9 cuts off the incident light from being transmitted into other parts than the photo diode. In order to allow the excessive electric charger to overflow into the N-type silicon substrate 1, a reverse bias voltage of the P-type wafer 2 is applied to the N-type silicon substrate 1 from a bias source 10. (This technique was disclosed, for example, in ISSCC Digest Technical Papers, pp. 168-169, 1982.)
FIG. 6 shows only one unit cell of the CCD solid state image sensor, but actually multiple photo detectors (PD) and charge reading parts (CCD) are arranged in a row in a direction orthogonal to the surface of FIG. 6. Furthermore, in a two-dimensional solid state image sensor, multiple rows of photo detector (PD) and charge reading part (CCD) units are provided.
In such a configuration, when light enters the photo diode through the silicon dioxide film 7 through the portion not covered by the aluminum film 9, electric charges are generated in the vicinity of the N-type region 5 and the P-type well 2 which together compose the photo diode. The electric charges are read out into the N-type region 6 of the CCD by applying a reading potential to the polycrystalline silicon electrode 8, and then, by alternately applying a high potential and low potential to the multiple polycrystalline silicon electrodes 8 arranged in a direction orthogonal FIG. 6 surface of the that is, the electric charges accumulated in the N-type region 6 are sequentially transferred in a direction orthogonal to the surface of the FIG. 6.
Furthermore, in the conventional CCD solid state image sensor shown in FIG. 6, some output current is observed even if the incident light is cut off completely. This kind of noise is called dark current. Dark current is known to increase exponentially with a rise in temperature. Further, the following has been discovered during experiments by the present inventors. The image lag characteristic is known as being one of the important characteristics of the CCD solid stage image sensor. The image lag characteristic is determined according to the structure of the photo diode. Therefore, usually, the image lag is suppressed by using a so-called complete depletion type photo diode. It was, however, found that the dark current increases when the photo diode of the complete depletion type is used in order to improve the image lag characteristic.
When the dark current increases, the ratio of the dark current (N) to signal current (S) increases, particularly at a low illumination intensity, and the so-called, S/N ratio deteriorates. Additionally, if the amplitude of the dark current varies in each photo diode, the fixed pattern noise increases when the illumination intensity is low or the temperature is high.
Therefore, in order to improve the overall characteristics of the CCD solid state image sensor, it is extremely important to suppress the dark current itself and the fluctuations in the amplitude of the dark current in multiple photo diodes.